This invention relates to the purification of alkali metal hydroxides and more in particular to the removal of soluble impurities from alkali metal hydroxides by contacting such hydroxides with ammonia.
Alkali metal hydroxides, such as sodium hydroxide, are generally produced commercially in the so-called mercury cells or diaphragm cells. In sodium hydroxide produced by mercury cells, a sodium chloride impurity may be present in an amount of roughly 0.01 to 0.001 weight percent. A typical diaphragm cell aqueous effluent contains about 8 to 10 weight percent sodium hydroxide and 15 weight percent sodium chloride. The diaphragm cell effluent is concentrated by evaporation to raise the sodium hydroxide concentration to about 50 weight percent and to reduce the sodium chloride concentration. However, even after such reduction of the sodium chloride, the 50 percent aqueous sodium hydroxide solution from the diaphragm cell still contains about 1 to 2 weight percent sodium chloride.
A number of processes have been developed throughout the years to reduce the sodium chloride concentration in aqueous sodium hydroxide. Many patents have issued describing the removal of sodium chloride and sodium chlorate from aqueous solutions containing a 50 percent concentration of caustic soda. U.S. Pat. No. 2,196,594 discloses an apparatus and process to purify a 50 percent or greater concentration of caustic liquor by countercurrently contacting the aqueous caustic liquor with liquid ammonia at a superatmospheric pressure and temperatures of 50.degree. to 100.degree. C. U.S. Pat. Nos. 2,196,595; 2,285,299; 2,285,300; 2,349,596; 2,325,339; 2,354,823 and 2,373,257 disclose still other processes for treating an aqueous sodium hydroxide containing solution with ammonia. The process of U.S. Pat. No. 2,622,009 removes salt impurities from an aqueous caustic soda solution, containing 45 to 50 percent caustic, using liquid ammonia and contact temperatures of 130.degree. to 160.degree. F. at pressures above 300 pounds per square inch (psi). These processes have been successful in eliminating a major portion of the sodium chloride impurity, but the existing ammonia-aqueous sodium hydroxide impurity extracting processes have generally not been able to consistently reduce the sodium chloride impurity in the caustic to a concentration level closely approaching that of mercury cell caustic.
Consequently, it is desired to provide a process which can reduce the impurity (such as sodium chloride) concentration in an aqueous alkali metal hydroxide solution to a level approaching that of mercury cell caustic.